The invention relates to a television camera tube comprising, in an evacuated envelope, an electron gun for generating an electron beam. During operation of the tube, the electron beam, is focused to form a spot on a photosensitive target. On the target, a potential distribution is formed by projecting an optical image on it. By scanning the target with an electron beam, signals corresponding to the said optical image are produced. The scanning takes place in a line deflection direction and a frame deflection direction.
The photosensitive target usually consists of a photoconductive layer which is provided on a signal plate. The potential distribution, sometimes called a potential image, is formed because the photoconductive layer may be considered to be composed of a large number of picture elements. Each picture element in turn may be considered as a capacitor to which a current source is connected in parallel the current strength of which is substantially proportional to the light intensity on the picture element. Hence the charge on each capacitor decreases linearly with time at constant light intensity.
As a result of the scanning, the electron beam passes through each element periodically and again charges the capacitor, which means that the voltage across each picture element is periodically brought to the potential of the cathode. The quantity of charge which is necessary periodically to charge one capacitor is proportional to the light intensity on the relevant picture element. The associated charge current flows via a signal resistance to the signal plate which all picture elements have in common. As a result of this, a voltage variation arises across the signal resistor, which voltage as a function of time represents the light intensity of the optical image as a function of the target location.
A television camera tube of the type is called a vidicon. A vidicon type television camera tube is known from the publication "Een experimentele kleine kleurentelevisiecamera" (An experimental small color television camera) in Philips Technisch Tijdschrift, Volume 29, 1968, No. 11.
In television camera tubes of the vidicon type the current density distribution in the electron beam is rotationally symmetrical at least up to a certain distance from the axis of the tube. The spot formed by the electron beam on the target may be considered as an electron-optical display of the smallest cross-section of the beam from the electron gun. The smallest cross-section of the beam occurs at either a cross-over, or a small circular bore sometimes called a diaphragm.
The display of this smallest beam cross-section is produced by rotationally symmetrical electrostatic and/or magnetic fields so that the current density distribution in the spot on the target is also rotationally symmetrical. A disadvantage of this rotationally symmetrical distribution in the spot is that upon scanning an optical image having a periodic pattern the modulation depth depends considerably on the orientation of the pattern relative to the line and frame deflection directions.
The modulation depth is a measure of the resolving power of the television camera tube and is defined as the ratio between the largest and the smallest value of the amplitude of the signal current upon scanning a given test pattern. The test pattern generally consists of vertical (perpendicular to the line deflection direction) light bands separated by equally wide dark bands. In some parts of the target the width of the band is such that approximately 20 pairs of light and dark bands could fill a complete picture height. In television technology this is called 40 "lines". In the remaining parts of the display screen this number is 200 pairs (that is 400 "lines"). The system of bands is scanned in the line deflection direction. When scanned by the electron beam, this text pattern provides a signal current which is an alternating current with respective fundamental frequencies of 0.5 and 5 MHz. These frequency values apply to a system of 625 lines per frame and a frame period of 1/25 second. For systems having a smaller or a larger number of lines and/or different frame periods, corresponding test patterns are possible.
The modulation depth is the value expressed in percent of the ratio of the amplitude of the 5 MHz signal and the 0.5 MHz signal. This measuring method is described in detail in the publication "Het plumbicon, een nieuwe televisie-opneembuis" (The plumbicon, a new television camera tube), Volume 25, 1963, No. 9). Upon rotation of such a test pattern with unvaried width of the bands relative to the deflection direction, the modulation depth as a function of the angle .alpha. (.alpha. being the angle between the direction of the bands of the test pattern and the frame deflection direction) proves to have an asymmetrical variation in whih a rotation of the test pattern to the right viewed from the camera tube will be considered as positive and a rotation to the left will be considered as negative. It is also assumed that the scanning takes place from the left to the right and from the top to the bottom of the frame. With negative angles .alpha., a rather strong decrease of the depth of modulation occurs relative to the usual position of the test pattern (which we define as .alpha.=0.degree.), With positive angles .alpha., the modulation depth initially increases and then decreases slowly only at large values of .alpha.. It will be obvious that this nonsymmetrical strong dependence of the modulation depth on the orientation of the test pattern is not desired.